I guess it should look something like below. Just provide your service princpal id:

from databricks.sdk.service.pipelines import RunAs
run_as_instance = RunAs(service_principal_name="your_service_principal_id")

  w.pipelines.update(
        pipeline_id=pipeline_id,
        name=pipeline.name,
        libraries=pipeline.spec.libraries,
        catalog=pipeline.spec.catalog,
        target=pipeline.spec.target,
        configuration=configuration,
        development=pipeline.spec.development,
        edition=pipeline.spec.edition,
        serverless=pipeline.spec.serverless,
        run_as = run_as_instance
    )